This painting is called "Milk and cookies" but I'm going to guess that many who see it will wonder why the milk isn't white. Everyone knows that milk is white and the milk in this painting doesn't look white. But I know that - I painted it! Why didn't I make the milk white? Here's why.
Milk isn't white.
What?? That's right. Milk isn't white. Milk only looks white sometimes - maybe lots of the time - but milk, like everything else, looks the way it looks in comparison to all the things around it. In the photo below you can see the actual glass of milk that was sitting in front of me when I made my little painting. As you can see, my painting does a pretty good job of matching the color of that glass of milk. It's mostly gray-green. But, since we "know" that milk is white, our brain sorts it out for us and we "see" a glass of white milk, even though we don't actually see it.
If you're going to paint something, don't paint what you "know" -- paint what you actually see when you're really looking at it.